1. Field of the Invention
The present invention relates to an alarm apparatus for indicating occurrence of abnormality in an alternating current generator mounted on a motor vehicle (hereinafter also referred to as the vehicle-onboard AC generator) which includes a pair of armature coils. More particularly, the invention is concerned with an alarm apparatus for a vehicle-onboard AC generator which apparatus can ensure an improved alarm function owing to a circuit arrangement for detecting the output voltage from each of the paired armature coils.
2. Description of Related Art
For a better understanding of the concept underlying the present invention, description will first be made of a hitherto known or conventional alarm apparatus for a vehicle-onboard AC generator which includes a pair of armature coils by reference to FIG. 3 which is a circuit diagram showing a structure of the conventional alarm apparatus.
Referring to FIG. 3, an AC generator 1 mounted on a motor vehicle (not shown) is adapted to be driven by an engine of the motor vehicle, i.e., vehicle-onboard engine (not shown either), and includes a field coil 10 and a pair of armature coils 11 and 12.
Each of the armature coils 11 and 12 of the AC generator 1 is constituted by three phase windings. Further, the armature coils 11 and 12 are disposed with a phase difference of e.g. 30.degree. in terms of electrical angle relative to a field coil 10 for the purpose of suppressing ripple components of the output voltage of the AC generator as a whole.
A pair of rectifier circuits 21 and 22 are connected to the AC generator 1 so that output voltages generated by or induced in the armature coils 11 and 12, respectively, can undergo full-wave rectification.
Each of the rectifier circuits 21 and 22 is constituted by diode pairs (i.e., pairs of diodes) connected in parallel in correspondence to individual phase windings, respectively, of each of the armature coils 11 and 12.
Further, the rectifier circuits 21 and 22 are connected in parallel with each other and equipped with positive (plus) output terminals 21a and 22a located at the cathode side of the individual diode pairs and negative (minus) output terminals 21b and 22b at the anode side of the individual diode pairs.
A voltage regulator (VR) 3 connected to one end of the field coil 10 is comprised of a grounded-emitter power transistor 31 for regulating or adjusting a field current flowing through the field coil 10, a suppression diode 32 connected to the collector of the power transistor 31, and a comparator 33 for driving the power transistor 31 to ON-state (conducting state). The comparator 33 may be constituted by a differential amplifier known per se.
The comparator 33 has an inversion input terminal (-) to which a voltage appearing at the positive or plus output terminals 21a and 22a of the rectifier circuits (or voltage appearing at a plus electrode of a battery described hereinafter) is applied, while a comparison reference voltage VR1 is applied to the non-inversion input terminal (+) of the comparator 33.
Connected to one end of the armature coil 11 is an alarm circuit 4 which is comprised of a resistor 41 serving as a constant-voltage power source 40, a Zener diode 42 connected in series to the resistor 41, a grounded-emitter alarm output transistor 43 and a comparator 44 for driving the alarm output transistor 43 to ON-state (conductive state). The comparator 44 may equally be constituted by a differential amplifier known per se.
Further, the alarm circuit 4 includes a parallel circuit of a smoothing condenser 45 and a resistor 46 inserted between the inversion input terminal (-) of the comparator 44 and the ground potential, and a series circuit of a resistor 47 and a reverse-current blocking diode 48 inserted between the inversion input terminal (-) of the comparator 44 and the one end of the armature coil 11.
Additionally, the alarm circuit 4 includes an alarm indicator lamp 49 connected to the collector of the alarm output transistor 43. The alarm indicator lamp 49 is driven to the lit state (electrically energized state) when the alarm output transistor 43 is driven to the ON-state.
Applied to the inversion input terminal (-) of the comparator 44 is an output voltage from one end of the armature coil 11 by way of the resistor 47 and the reverse-current blocking diode 48, while applied to the non-inversion input terminal (+) of the comparator 44 is a comparison reference voltage VR2.
A battery 5 is adapted to be charged with the rectified output power fed from the positive (plus) output terminals 21a and 22a of the rectifier circuits and supply electric power to the AC generator 1, the voltage regulator (VR) 3 and the alarm circuit 4.
Inserted between the plus electrode of the battery 5 and the alarm circuit 4 is a key switch 6 which is turned on or closed upon starting of operation of the motor vehicle.
The voltage appearing at the plus electrode of the battery 5 is applied to the other end of the field coil 10 of the AC generator 1, the cathode of the suppression diode 32 which constitutes a part of the voltage regulator (VR) 3, the inversion input terminal (-) of the comparator 33 constituting another part of the voltage regulator (VR) 3, the resistor 41 of the alarm circuit 4 and the alarm indicator lamp 49, respectively.
The power transistor 31 of the voltage regulator (VR) 3 is turned on in response to the output of the comparator 33 when the rectified output voltages of the rectifier circuits 21 and 22 or the terminal voltage of the battery 5 is lower than the comparison reference voltage VR1 inclusive thereof, whereby the current flowing through the field coil 10 is so controlled by the voltage regulator (VR) 3 that the output voltages of the armature coils 11 and 12, respectively, are regulated to be equal to a level corresponding to the comparison reference voltage VR1.
On the other hand, the alarm output transistor 43 of the alarm circuit 4 is turned on when the output voltage appearing at one end of the armature coil 11 (i.e., voltage of one phase) becomes lower than the comparison reference voltage VR2 inclusive, whereby the alarm indicator lamp 49 is driven to the lit-state, i.e., alarm generating state.
Next, description will turn to operation of the conventional alarm apparatus for the vehicle-onboard AC generator shown in FIG. 3.
Immediately after the key switch 6 is turned on, operation of the vehicle engine is not started yet. Thus, the AC generator 1 is in the non-driven state. Consequently, no output voltage is generated by the armature coils 11 and 12.
Under the circumstances, the voltage applied to the inversion input terminal (-) of the comparator 44 of the alarm circuit 4 remains lower than the comparison reference voltage VR2 inclusive. Thus, the comparator 44 generates the comparison output of level "H".
As a result of this, the alarm output transistor 43 is turned on, whereby the alarm indicator lamp 49 is electrically energized to be lit.
When the power generation of the AC generator 1 is started in succession to the start of operation of the vehicle-onboard engine, the output voltage of the armature coils 11 and 12 rises or increases. When the voltage applied to the inversion input terminal (-) of the comparator 44 exceeds the comparison reference voltage VR2, the comparator 44 generates the output signal of level "L".
Consequently, the alarm output transistor 43 is turned off, whereby the alarm indicator lamp 49 is electrically deenergized, indicating that a normal power generation state is prevailing.
On the other hand, the voltage regulator (VR) 3 detects the terminal voltage of the battery 5 (or rectified output voltages of the rectifier circuits 21 and 22, respectively), to thereby perform on-off control of the power transistor 31 and hence the current flowing through the field coil 10 with the aid of the comparison output signal of the comparator 33 so that the output voltage of the AC generator 1 can be regulated to the level which coincide with the comparison reference voltage VR1.
In that case, a surge voltage may be induced in the field coil 10 due to on-off operation of the power transistor 31. However, such surge voltage can be absorbed by the suppression diode 32.
When the output voltage of the armature coils 11 and 12 lowers below the comparison reference voltage VR2 for some reason even in the normal operation state of the AC generator 1, the comparator 44 of the alarm circuit 4 generates the comparison output signal of level "H".
Consequently, the alarm output transistor 43 is turned on to thereby energize electrically the alarm indicator lamp 49, messaging an abnormal power generation state of the AC generator.
As is apparent from the foregoing, in the case of the conventional alarm apparatus for the vehicle-onboard AC generator known heretofore, the alarm circuit 4 is connected to only one of the paired armature coils (i.e., to the armature coil 11) for detecting the output voltage thereof. Consequently, it is impossible to detect the change or lowering of the output voltage of the other armature coil 12, giving rise to a problem that the alarm function for abnormal lowering of the generated voltage can not be ensured adequately.